Retractable propulsors for boats are known, for example, from the French patent No. 2.229.608.
This document describes an auxiliary propulsor in which the propeller is withdrawn into a cavity forseen under the hull. The propeller is fixed to a shaft which penetrates an upper hollow tube when the propeller is raised. The lifting is carried out by means of a hydraulic jack, one end of which is fast with to a fixed frame and the other to a horizontal plate placed under the propeller. For the rotation of the assembly telescopic elements are housed in another tube fixed to the craft on which the same rotate. The pressure oil contained in the transmission system is used for controlling the hydraulic jack and for rotating the driving shaft.
The main disadvantages which are noted in this embodiment are the lack of mechanical sturdiness, as well as the general kinematic complication due to the fact that the whole assembly must rotate, the possibility of using only hydraulic engines and on the whole the considerable size. Another disadvantage is that of finding the optimal application only on flat-bottomed boats.